AM Stereo phase modulation decoder

Abstract

A decoder for obtaining the L-R information in an AM stereo radio receiver. The decoder is basically a four-quadrant multiplier that has one pair of inputs driven from a limiter that operates from the receiver intermediate frequency amplifier. The limiter also drives a tuned circuit which in turn drives the other two multiplier inputs in phase quadrature with the first input pair. The multiplier output is frequency modulation responsive which, when integrated, produces a phase modulation response. The integration is produced by a capacitor connected across the multiplier output terminals. The decoder also includes a large parallel connected inductor that resonates the integrating capacitor at a frequency at the low end of the audio range. This inductor acts as a d-c short at the decoder output and sets the decoder response to the setaudible stereo pilot signal. The inductor is simulated with integrated circuit components. A first transconductance amplifier coupled to the multiplier output, drives a capacitor which in turn drives a second transconductance amplifier that operates current sources which act as load elements for the multiplier. The resultant feedback loop creates an inductive reactance that simulates a large inductor.

I claim: 
     
       1. A phase demodulator comprising: a four quadrant multiplier having first and second input terminal pairs and an output terminal pair;   means for driving said first input pair from a source of phase modulated signal;   means for coupling said phase modulated signal in said multiplier to a tuned circuit resonating at the center frequency of said signal whereby said tuned circuit is caused to oscillate;   means for coupling said tuned circuit to said second input pair to induce a quadrature signal in said multiplier thereby to create a frequency sensitive response;   a capacitor coupled across said output terminal pair for integrating the signal at said output terminal pair thereby to produce a phase responsive output; and   first and second variable current sources connected as load elements to said output terminal pair of said multiplier.   
     
     
       2. The demodulator of claim 1 further comprising feedback means operatively coupled in common to control said first and second current sources and responsive to the common mode voltage at the output of said multiplier. 
     
     
       3. The demodulator of claim 1 wherein said demodulator further comprises an inductor coupled in parallel with said capacitor to resonate said output terminal pair at a frequency located at the low end of the audio frequency spectrum. 
     
     
       4. The demodulator of claim 3 wherein the value of said inductor is selected to determine the circuit response to a subaudible frequency carrier modulation signal. 
     
     
       5. The demodulator of claim 3 wherein said inductor is simulated in the form of a circuit comprising: a first transconductance amplifier having a pair of inputs coupled to said output terminal pair and a current source output;   a capacitor coupled to said current source output whereby the voltage across said capacitor lags said current in phase by approximately 90 degrees;   means coupling the voltage across said capacitor to the input of a second differential transconductance amplifier which has a pair of differential output terminals; and   means coupling said pair of differential output terminals of said second transconductance amplifier to separately control said first and second variable current sources whereby the current signals fed back to said multiplier output terminal pair lag the voltage therebetween by approximately 90 degrees to simulate an inductor.   
     
     
       6. The demodulator of claim 5 further comprising a buffer amplifier having a pair of input terminals coupled to said output terminal pair of said four quadrant multiplier and a pair of output terminals coupled to said input terminals of said first transconductance amplifier. 
     
     
       7. The demodulator of claim 6 further comprising current comparator means with deadband coupled to said output terminals of said buffer amplifier whereby said deadband accomodates the normal tuning range of said demodulator and produces an output when said tuning range is exceeded.